error[E0658]: `Box<Struct, impl FnMut(&mut Self)>` cannot be used as the type of `self` without the `arbitrary_self_types` feature
  --> $DIR/could-not-resolve-issue-121503.rs:6:35
   |
LL |     async fn box_ref_Struct(self: Box<Self, impl FnMut(&mut Self)>) -> &u32 {
   |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: see issue #44874 <https://github.com/rust-lang/rust/issues/44874> for more information
   = help: add `#![feature(arbitrary_self_types)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0658`.